Intermodulation product cancellation circuit

• US 6,172,564 B1
• Filed: 07/30/1999
• Issued: 01/09/2001
• Est. Priority Date: 07/30/1999
• Status: Expired due to Term

• Current Assignee: Omniphase Research Laboratories, Inc.
• Original Assignee: Eugene Rzyski
• Inventors: Rzyski, Eugene
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Choe, Henry
• Application Number:
  US09/363,895
• Time in Patent Office: 529 Days
• Field of Search: 330/149, 375/296, 375/297, 455/126
• US Class Current: 330/149
• CPC Class Codes: H03F 1/3223 using feed-forward H03F1/3211 takes precedence

Abstract

A cancellation circuit is provided for canceling undesirable intermodulation (IM) products in electrical circuitry. The IM cancellation circuit includes a stepping spectrum analyzer which recognizes and measures the frequencies and amplitudes of different signal components of an RF signal across a desired frequency band. This information is forwarded to a processor which contains look-up tables or the like which provides information relating to the frequencies and/or amplitudes of signal components which are intended to reside in the RF signal. By comparison of the look-up tables to the frequencies and amplitudes measured and recognized by the sweeping spectrum analyzer, the processor of the cancellation circuit is able to determine whether the signal components in the RF signal are intended “information” carrying signals or unintended IM products. Where the processor has determined that a signal component is an unintended IM product, the processor controls a variable voltage controlled oscillator and variable amplifier to create a cancellation signal having an identical frequency and amplitude as the frequency and amplitude of the IM product. A phase shifter is provided to shift the phase of the cancellation signal 180° with respect to the IM product and the cancellation signal is combined with the RF signal to produce an output signal having the IM product canceled.

Claims

1. An intermodulation (IM) product cancellation circuit for reducing one or more IM products in a carrier signal, the IM product cancellation circuit comprising:

a variable oscillator for producing an IM product cancellation signal;

a variable phase shifter means for shifting said IM product cancellation signal 180°

with respect to an IM product in a carrier signal for canceling an unwanted IM product in the carrier signal;

a processor including a memory for storing a look-up table including the identity of the frequency or frequencies of intended signal components of a carrier input signal, said processor connected to said variable oscillator and said variable phase shifter for controlling said variable oscillator and said variable phase shifter means for initiating and maintaining said IM product cancellation signal at substantially the same frequency as the IM product but shifted 180°

with respect to the IM product for substantially reducing an unwanted IM product in the carrier signal; and

a coupler means for combining the carrier signal with said IM product cancellation signal to produce an output signal the IM product being substantially reduced.

2. The IM product cancellation circuit of claim 1 further comprising an amplifier for controlling the amplitude of said IM product cancellation signal.

3. The IM product cancellation circuit of claim 2 wherein said amplifier is a variable amplifier which is connected to said processor.

4. The IM product cancellation circuit of claim 3 wherein said processor adjusts said variable amplifier so that said IM product cancellation signal has substantially the same amplitude as said IM product for substantially canceling the IM product in the carrier signal.

5. The IM product cancellation circuit of claim 1 further comprises a spectrum analyzer connected to said processor for recognizing and measuring the characteristics of within a desired frequency band.

6. The IM product cancellation circuit of claim 5 wherein said spectrum analyzer is capable of automatically stepping across a desired frequency band for recognizing and measuring the characteristics of a carrier signal within that frequency band.

7. The IM product cancellation circuit of claim 5 wherein said processor includes a look-up table providing information regarding the desired characteristics of said carrier signal for enabling the processor to determine the existence of unwanted IM products by comparison of the results of the analysis by said spectrum analyzer to information stored in said look-up table.

8. The IM product cancellation circuit of claim 6 wherein said processor includes a look-up table providing information regarding the desired characteristics of said carrier signal for enabling the processor to determine the existence of unwanted IM products by comparison of the results of the analysis by said spectrum analyzer to information stored in said look-up table.

9. The IM product cancellation circuit of claim 7 wherein in said cancellation circuit is provided to cancel IM products of a primary amplifier.

10. The IM product cancellation circuit of claim 9 wherein information stored in said look-up table is provided by analysis by said spectrum analyzer of said carrier signal prior to said carrier signal entering said primary amplifier.

11. The IM product cancellation circuit of claim 1 further comprising:

an analog-to-digital converter for converting a portion of the carrier signal into a digital format for analysis and measurement of signal components in the carrier signal.

12. The IM product cancellation circuit of claim 1 wherein said circuit comprises a plurality of variable oscillators, variable amplifiers, and variable phase shifters for producing a corresponding plurality of IM product cancellation signals for canceling a plurality of IM products.

13. A method of canceling unwanted IM product in a carrier signal comprising the steps of:

providing a look-up table including the identity of the frequency or frequencies of intended signal components of a carrier input signal;

sampling the carrier signal to produce a carrier signal sample;

analyzing the carrier signal sample to determine if the carrier signal has an unwanted IM product by comparing the analysis of the carrier signal sample to the look-up table including the frequency or frequencies of intended signal components;

controlling a variable oscillator to produce an IM product cancellation signal having substantially the same frequency as the unwanted IM product;

shifting the IM product cancellation signal 180°

with respect to the IM product in the carrier signal; and

combining the carrier signal with the IM product cancellation signal to produce an output signal with the unwanted IM product being substantially canceled by the IM product cancellation signal.

14. The method of canceling unwanted IM products in a carrier signal of claim 13 further comprising the step of:

amplifying the IM product cancellation signal to have substantially the same amplitude as the unwanted IM product prior to combining the carrier signal with the IM product cancellation signal.